b.chk_map : module documentation

The CHKMap class implements a dict from tuple_of_strings->string by using a trie
with internal nodes of 8-bit fan out; The key tuples are mapped to strings by
joining them by , and padding shorter keys out to the length of the
longest key. Leaf nodes are packed as densely as possible, and internal nodes
are all an additional 8-bits wide leading to a sparse upper tree.

Updates to a CHKMap are done preferentially via the apply_delta method, to
allow optimisation of the update operation; but individual map/unmap calls are
possible and supported. Individual changes via map/unmap are buffered in memory
until the _save method is called to force serialisation of the tree.
apply_delta records its changes immediately by performing an implicit _save.